The present invention relates to retroreflective articles that exhibit good retroreflective brightness under wet and dry conditions and good retroreflective brightness at high entrance angles.
Pavement markings, such as those on the centerline and edgeline of a roadway, are important in order to provide visual guidance for motor vehicle drivers. Pavement marking materials are used as traffic control markings for a variety of uses, such as short distance lane striping, stop bars, and pedestrian pavement markings at intersections and long line lane markings, etc. on roadways. A common form of pavement marking material is adhesive-backed tape that is applied to the roadway surface in desired location and length; the top surface of the tape having selected color and typically retroreflective characteristics.
Currently, many flat pavement markings typically rely on an exposed-lens optical system comprising transparent microspheres partially embedded in a binder layer containing reflective pigment particles such as titanium dioxide (TiO2) or lead chromate (PbCrO4) as the retroreflector mechanism. In use, light from the headlamp of a vehicle enters the microsphere and is refracted to fall on the reflective pigment. Some portion of the light is returned generally along the original entrance path in the direction of the vehicle so as to be visible to the driver. The amount of refraction and the amount of light gathering of these microspheres is dependent in part upon maintaining a low index of refraction air interface on the exposed portion of the microsphere. During rainy periods, the microspheres become wetted with water which reduces their light refracting ability resulting in much reduced retroreflective performance.
It is also known to use enclosed-lens retroreflective structures on pavement markings. See, e.g., U.S. Pat. No. 4,875,798 (May) and U.S. Pat. No. 5,340,231 (Steere et al.). Enclosed-lens retroreflective sheetings with flat cover films (also sometimes referred to as top films, top sheets, cover sheets, etc.) have been constructed as a means for providing effective retroreflection under wet conditions. See, e.g., U.S. Pat. No. 4,025,159 (McGrath) which discloses encapsulated-lens retroreflective articles and U.S. Pat. No. 4,505,967 (Bailey) and U.S. Pat. No. 4,664,966 (Bailey et al.) which disclose embedded-lens retroreflective articles.
Currently available low profile pavement markings provide effective retroreflective response for only a narrower range of entrance angles than is sometimes desired. For example, flat pavement markings, relying on microspheres partially embedded in layers containing diffuse pigments as described above, are most easily seen at distances of approximately 80 meters and less. At distances greater than this, retroreflective brightness declines due in part to the relatively larger entrance angles of the incident light and in part to inherently limited retroreflective brightness. In addition to generally low retroreflectivity at high incidence angles, flat pavement markings are particularly difficult to see under rainy conditions for the reasons discussed above. Raised pavement markers typically have better wet reflectivity because the rain will run off the raised portions and they sometimes use reflective systems that are inherently retroreflective when wet. However, snow removal is frequently a problem on roads bearing raised pavement markers, as the snowplows have a tendency to catch on the raised protrusions and dislodge the markers from the road surface. Also, raised pavement markers mounted as spot delineators provide relatively poor daytime road delineation and thus commonly need to be augmented with continuous painted or tape line markings.
To expand the effective entrance angle range of a pavement marking material, combination of a light directing means with an enclosed-lens retroreflector has been proposed. For example, U.S. Pat. No. 4,145,112 (Crone) discloses an article comprising an underlying base retroreflective layer and a light directing layer comprised of a longitudinally-extending series of short transparent projections. U.S. Pat. No. 4,236,788 (Wyckoff) discloses a related type of pavement marker strip wherein the two sides of the transverse prisms are adjusted to provide for downward internal reflection into the base sheet from one side and refraction to the space between successive prisms into the base sheet from the other side. U.S. Pat. No. 3,920,346 (Wyckoff) discloses a saw-tooth-like marker strip comprising protrusions with curved edges and having upwardly disposed retroreflective members embedded therein.
U.S. Pat. No. 4,072,403 (Eigenmann) discloses a retroreflective assembly that is particularly useful for situations in which retroreflection is required in rainy conditions. The assembly disclosed therein comprises a transparent globule with a layer of transparent microspheres on certain portions of the globule and a reflective layer disposed behind the microspheres. The retroreflective assemblies, sometimes referred to as xe2x80x9cglobule/microsphere retroreflective assembliesxe2x80x9d, are positioned on the top surface of a pavement marking where they provide effective retroreflection of light at high incidence angles. U.S. Pat. No. 5,268,789 (Bradshaw) discloses an improved retroreflective assembly of this type and an improved method for making such an assembly.
EP Patent publication 385746 B1 (Kobayashi et al.) discloses a high-brightness all-weather pavement marking. The marking includes glass microspheres of relatively large diameter partially embedded in a first transparent resin layer that serves to hold the microspheres. The pavement marking further includes glass microspheres of relatively small diameter that are buried in a second transparent resin layer. In the lower portion of the second transparent resin, behind the rear of the small glass microspheres, lies a reflecting layer. In use, incident light strikes the large glass microspheres, travels through the second transparent resin layer, strikes the small glass microspheres, is reflected by the reflecting layer, and returns in the direction from which it came, i.e., through the small glass microspheres, through the second transparent resin layer, and finally through the large glass microspheres. The small glass microspheres and large glass microspheres work together to retroreflect incident light. It is also described that the large glass microspheres, being partially exposed in air, can receive a large amount of incident light, even if such light is at a large incidence angle. It is further described that the pavement marking is useful in rainy conditions.
There still exists a need for a low profile retroreflective article that exhibits good retroreflective characteristics under dry and wet conditions, preferably at high incidence angles in both daytime and nighttime, possesses suitable skid resistance, and is suitably durable.
Applicants have found that an article having a combination of exposed-lens retroreflective elements and enclosed-lens retroreflective sheeting provides retroreflectivity under dry and dynamic wet conditions at very high entrance angles (e.g., 80xc2x0 or more, preferably 85xc2x0 or more). The inventive articles are well suited for use on vertical and horizontal surfaces, but are particularly beneficial for use on horizontal surfaces, such as roadway pavement. The invention also provides a method for making such materials. Much of the following description is made with specific reference to embodiments of the invention to be used as pavement marking articles.
One aspect of the present invention provides a retroreflective article including an enclosed-lens retroreflective sheeting having a first surface and a second surface, wherein, under dry conditions, the sheeting exhibits a coefficient of retroreflective luminance at a simulated 30 meter viewing geometry as measured by ASTM D 4061-94 of about 40 (millicandela/m2)/lux, and under dynamic wet conditions, the sheeting exhibits an increase in coefficient of retroreflective luminance; a light transmissible bonding layer disposed on the first surface of the retroreflective sheeting; and a plurality of retroreflective elements partially embedded in the bonding layer.
In accordance with the present invention, the enclosed-lens retroreflective sheeting is preferably selected from the group consisting of microsphere type sheeting and cube-corner type sheeting. The retroreflective sheeting can be selected from the group consisting of an embedded-lens retroreflective sheeting and an encapsulated-lens retroreflective sheeting.
Preferably, the retroreflective elements are substantially spherical in shape, ranging in a size from about 0.5 mm to about 1.2 mm. In one embodiment, the retroreflective elements cover from about 0.4% to about 6.6% of a surface area of the retroreflective article. Preferably, the microspheres used in the retroreflective elements have a refractive index of about 1.50 to about 2.60. The retroreflective elements preferably comprise ceramic optical elements partially embedded in an opacified glass core. The ceramic optical elements can comprise an amorphous phase, a crystalline phase, and a combination thereof. Preferably, the core comprises at least two phases, one phase in a size range from about 0.05 micrometers to about 1.0 micrometers. The core can include at least two phases, wherein one phase has a refractive index value of at least 0.4 greater than the second phase. Preferably, the opacified glass comprises an opacifier having a refractive index of about 1.9 to about 2.7.
The retroreflective article in accordance with the present invention may further include a layer of adhesive on the second surface of the retroreflective article. The article may also include at least one of a conformance layer and a scrim layer on the second surface of the retroreflective article.
Preferably, the light transmissible bonding layer is formed from a polyurethane composition. The retroreflective article may also include a top film disposed on a surface of the bonding layer opposite the retroreflective sheeting or disposed directly on the retroreflective sheeting""s first surface. Preferably, the enclosed-lens retroreflective sheeting includes a layer of microspheres embedded in a light transmissible bonding composition and a specular layer in optical association with the microspheres, wherein a spacing layer separates the microspheres from the specular layer.
Another aspect of the present invention provides a method for making a retroreflective article including providing a retroreflective sheeting comprising a first surface and a second surface; applying a bonding composition to the first surface of the retroreflective sheeting, wherein the bonding composition is capable of forming a light transmissible bonding layer; and applying a plurality of retroreflective elements to the bonding composition so that the retroreflective elements are partially embedded in the bonding layer.
As used herein,
xe2x80x9cdryxe2x80x9d means that no additional moisture is added to any surface of a retroreflective article such that the only moisture in contact with the article is ambient moisture.
xe2x80x9cdynamic,xe2x80x9d when referring to wet conditions, means physical detectable movement of water is present on at least a portion of a surface of the retroreflective article. The physical movement of the water can be one of rain drops falling on the retroreflective article, wind blowing across the water surface on the retroreflective article, water draining off the surface of the retroreflective article, and the like, as well as a combination thereof. This dynamic condition is to be distinguished from a xe2x80x9cstatic-floodedxe2x80x9d water condition that is typified by a glass- or mirror-like surface of the water, i.e., no physical movement of the water can be detected where the retroreflective article is substantially submerged in the water.
xe2x80x9ccoefficient of retroreflective luminance (herein xe2x80x9cRLxe2x80x9d)xe2x80x9d means the ratio of the luminance, xe2x80x9cL,xe2x80x9d of a projected surface to the normal illuminance at the surface on a plane normal to the incident light, expressed as millicandelas per square meter per lux ((mcd/m2)/lux), as described in ASTM E 808-94.
xe2x80x9cenclosed-lens retroreflective sheetingxe2x80x9d comprises a layer of optical elements, whether microspheres (commonly referred to as xe2x80x9cbeadsxe2x80x9d) or cube-corner elements, having at least one polymer in contact with the optical elements such that they are not exposed to ambient conditions. Such sheeting includes embedded-lens and encapsulated-lens retroreflective sheeting.
xe2x80x9chigh entrance angle,xe2x80x9d when referring to incident light, means light rays of approximately greater than 80xc2x0 from vertical, preferably greater than 85xc2x0, and most preferably between 86xc2x0 and 90xc2x0, such as may be generated by headlights on vehicles illuminating a pavement surface.
xe2x80x9clight transmissiblexe2x80x9d means that a particular layer or medium transmits light from about 400 to about 700 nanometers wavelength (i.e., in the visible spectrum) of the electromagnetic spectrum.